Evolution by natural selection lies at the heart of Evfit. Indeed, it is the premise upon which our assumptions and observations derive.
The idea of evolution is very simple: living things vary one from another over the generations. Daughters, for example, differ in small ways from their mothers. With each succeeding generation the degree of difference from the baseline mother will, in some cases, be greater.
Variation is not, however, evolution.
Evolution occurs through natural selection. That is, where some external stress impacts on the varying descendants to the extent that some descendants can reproduce more successfully than other descendants. The characteristics that gave those descendants the greater ability to reproduce will be transmitted to their descendants who, in their turn, will be better adapted to the stressful environment.
In the case of human evolution, the fossil and genetic evidence shows that humans and chimpanzees last shared a common ancestor about five million years ago. Paleoecologists tell us that one of the critical stressors four and five million years ago was a gradual decline in temperature. This climatic change led to the receding of the jungles in which the chimp/human ancestors lived and the replacement of those jungles by open savannah. Primates who could more readily stand and walk and so be vigilant for predators approaching through the tall grass were "naturally selected" over those who were less able to maintain an effective vigilance. (There were other advantages of bipedalism, but I have used this one to illustrate natural selection at work.)
Note that evolution is gradual. It took hundreds of thousands of years for our ancestors to evolve from being primarily tree dwellers to being primarily ground dwellers. Many lions would have had many feasts of human flesh until the early humans adapted physically to thrive in their new open environment. One million years is about 40,000 generations and this indicates the rate of physical change we need to keep in mind. Greater stresses may bring about more rapid evolution, but for Evfit purposes the 10,000 years (400 generations) since the first few tribes edged their way into settled agriculture with its physical drudgery and its dietary dependence on one or two starchy staples (wheat, rice, tubers) would not have been long enough to evolve our hunter-gatherer bodies into a different, more adapted physiology. Indeed, so many of our cultural diversions from the hunter-gatherer lifestyle have been designed to overcome or replace the pressures of natural selection.
For a clear, inspiring and accessible account of evolution, I recommend Linda Gamlin's 1993 book "Evolution", volume 85 in the Dorling Kindersley Eyewitness Guides.(ISBN 0-7513-61402) This book takes the reader through the history of the idea of evolution presenting evidence in an illuminating and logical way. With professor J Maynard Smith as editorial consultant, you would expect it to be good - and it is!